Particulate matter (PM2.5) has been linked to a range of serious respiratory and cardiovascular health problems. The nitroPAHs found in the (PM2.5) are formed during combustion processes or by either chemical or photochemical reactions of polycyclic aromatic hydrocarbon in polluted atmospheres. NitroPAHs have been identified in extracts of respirable particles collected from polluted urban air, diesel exhaust particles, automobile exhaust, coal fly ash and wood smoke. The nitroPAHs are typically less abundant in ambient air than PAHs and are found at concentrations in the range of pg/m3 to ng/m3. Nonetheless, some of them could be more mutagenic or carcinogenic in laboratory bioassays than the parent PAH. Thus, it is of great significance to understand their sources and transformations in the atmosphere in assessing environmental exposure and risks. Specifically, their transformations at the solid/air interface or in the liquid-like environment of the organic fraction of the combustion derived aerosols can have a significant impact on controlling their residence time in the environment. It is important to study the photochemistry in these two environments because their photodegradation can proceed by entirely different mechanisms depending on the reaction medium. We are proposing to utilize techniques, already developed in our laboratory, to study the photochemical transformation mechanisms of nitroPAHs adsorbed or absorbed into models of atmospheric particulate matter in order to provide some understanding of the fate of these contaminants in the atmosphere. As we have found with PAHs, that phototransformations at the solid/air interface can have a significant impact in controlling their residence time in the environment, and are thus important in the evaluation of the potential risks of these contaminants, as well as in the possible design of systems for their removal. The working hypothesis is that the physical and chemical properties of the particulate matter are determining factors in the reactivity of the excited states and intermediates participating in the photochemical transformations of these pollutants in that environment. In order to understand the phototransformation mechanism of adsorbed or absorbed nitroPAHs we will: (1) isolate/and characterize the principal stable photoproducts and determine their quantum yields and the effect of the nature of the solvent (polar, non polar, polar aprotic, hydrogen abstraction easiness), and of organic compounds found in the atmospheric aerosols on the product yields, (2) isolate and characterize the principal stable photoproducts produced on adsorbents that mimic the atmospheric particle matter such as inorganic oxides, and sulfate salts, and determine the effect of the physical and chemical properties of the surfaces of these solids (such as composition average pore diameter, surface coverage) on the products relative yields, and to compare their relative yields and distribution with those obtained in the different solvents. The effect of coadsorbed water and oxygen on the yields will also be examined and (3) identify and characterize the participating excited states and reactive intermediates in the phototransformation process occurring in solution and on the surfaces. Related to this aim is the determination of the effect of organic cosolutes encountered in the atmospheric aerosol on the reaction kinetics of the intermediates. The physical properties of the participating excited states and reactive intermediates will be supported by quantum mechanical calculations.